You use the Poke Transfer. After you get your national pokedex, you gain access to the Poke Transfer lab. You will need another DS/DSi/DSi XL with the Pokemon game you received Celebi in. Then on Pokemon Black talk to the head scientist at the Poke Transfer lab and he'll let you use the machine. Go to DS Download Play on your other Pokemon game's system and wait for it to download the Poke Transfer software. Then pick 6 Pokemon to transfer and play a little catching mini-game. Then tada you have Celebi. Make sure you put Celebi in your PC in your other DS game before starting the transfer though.
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Celebi isn't available in Pokemon Black and White. You have to transfer it into Pokemon Black and White or trade for one.
In order to get Celebi on Pokémon Black without an event, you will need to trade for one or you will have to use a cheating device such as an Action Replay in order to get Celebi.
leave celeby n your PC box not on Pokemon black or white and it should probably say no:1 items (Pokemon) and it will save the games
Well, the only way you can get celebi is to trade with a friend who has one
you can trade pokemon from heartgold to black but you can only transfer the pokemon in heartgold not trade it from black